noun A manner of speaking, a mode of expression peculiar to a language, language family, or group of people.
In English, idiom requires the indefinite article in a phrase such as "she's an engineer", whereas in Spanish, idiom forbids it.
noun A programming construct or phraseology that is characteristic of the language.
noun A language or language variety; specifically, a restricted dialect used in a given historical period, context, etc.
In the idiom of the day, they were sutlers, although today they'd probably be called vendors.
noun A phrasal expression that is unreliably understood literally, requiring pre-established cultural or domain-specific context to infer its intended meaning.
She often spoke in idioms, pining for salad days and complaining about pots calling the kettle black.
noun An artistic style (for example, in art, architecture, or music); an instance of such a style.